How to create object instances with unique properties in Blender?
Alt + D creates a linked duplicate. You could just duplicate using Shift + D. Then each duplicate has its own copy of the properties and modifications are just to that object.

Alternately if you already have the linked duplicates (and only want have unique text), then go to one that you want to have a unique text and click the number button (number of users of this property). This creates a new data block, which you can then have different text (and text format) assigned to it.